The present invention relates to shape cutting machines and more particularly, to shape cutting machines having improved tracing capabilities.
Shape cutting machines have been utilized for many years to cut predetermined patterns in metal plates by means of oxygen-fuel flames or plasma arcs. Typically, one or more torches which are adapted to issue such flames are secured to an assembly which is mounted for translation along a transverse beam. A pair of carriages are typically utilized to support the beam together with appropriate gas and electrical control devices and are effective to translate the beam longitudinally along a set of rails. A tracing assembly, which may be mounted on a cantilever portion of the transverse beam, is effective to follow a template or the like and generate electrical signals indicative of the line or edge of the template being followed. A servomotor is provided for driving the tracer holder assembly along the transverse beam in an orthogonal direction with respect to the rails.
Torch holder assemblies are mounted for translation on the transverse beam and are mechanically connected to the tracer holder assembly. Thus the torch holder assemblies are driven along the transverse beam by the servomotor mounted on the tracer holder assembly. Prior art flame cutting machines have generally utilized a single drive motor mounted on one of the carriages for translating both carriages and the transverse beam assembly longitudinally to enable cutting of metal plate or the like in the two-dimensional pattern. As a consequence of the center of gravity of the transverse beam assembly being noticeably displaced from the drive motor mounted on a carriage, the transverse beam assembly tends to twist or skew when driven by a single side mounted carriage drive and consequently, "squareness" between the beam and rails, etc. is lost. This skewing effect tends to degrade the ability of the tracing assembly to follow a given line or edge which results in less than desirable cutting performance. More specifically, upon a tracing head "seeing" or following a corner on a template or the like, the tracer holder assembly is caused to undergo a sharp change in direction to follow such corner and the resulting twisting or skewing causes the tracer assembly to overshoot or undershoot the line it is intended to follow. Oscillations may be induced in the beam and cause degradation of tracing performance. Thus, prior art flame cutting systems utilizing a single carriage drive have not been effective to enable a satisfactory degree of tracer accuracy during cornering at relatively high speeds which decreases the economic benefits potentially obtainable from flame cutting systems. For example, such prior art flame cutting systems have been limited to cutting speeds of approximately 50-60 i.p.m. and attempts to increase speeds have led to significant degradation of cutting performance.
One approach to the foregoing problems of improving flame cutting machine operation is to simply stiffen or rigidify the structural components of the system such as the transverse beam assembly, carriages, etc. Although such an approach will enable improved tracer and flame cutting machine operation, the cost thereof becomes exorbitant in comparison to the improved performance.
In addition to the foregoing attempts to improve flame cutting operation, it is well known to provide tracing systems with a "lead" compensation such that the tracer effectively views the line or edge ahead of its actual position and generates appropriate electrical signals for driving the system servomotors accordingly. By anticipating changes in the line or edge to be followed, improved machine operation will be attained; however, in the event the lead distance is too short, the tracer will tend to overshoot the line to be followed and should the lead distance be too long, the tracing assembly will tend to inscribe a corner. If the system is capable of following a radius no greater than the kerf radius the resultant part will have a sharp corner. However, even with lead compensation, adequately rigidified machine frames must be provided to enable the benefits of such compensation to be obtained.
Thus, traditional prior art approaches to improving cutting machine operation have been a compromise of system performance with rigidity which has limited the degree to which machine performance can be improved at any reasonable cost.
It has been proposed to provide drive motors with each carriage of a shape cutting machine and certain systems incorporating this concept have been constructed. Typically, such systems utilize extremely rigid machine frames and/or relatively expensive synchrosystems in which a phase locked loop circuit is utilized to control the phases of signals supplied to each motor and thereby maintain the motors in precise synchronism with one another. In addition, it has been found necessary with such prior art dual side drive systems to utilize a pair of precision rack and pinion drives for translation of the carriages in a longitudinal direction. The additional cost of such sophisticated electronic circuitry and precision racks renders these prior art dual side drive systems extremely expensive. Other cutting systems utilizing a motor for driving each of a pair of carriages are illustrated in U.S. Pat. Nos. 2,389,585 and 3,912,242. Each of the systems described therein is utilized for simply effecting longitudinal cuts in metal slabs and neither machine is capable much less suitable for use in following two dimensional templates as neither machine is provided with any mechanism for driving torch assemblies along a transverse beam. Furthermore, each assembly described in the foregoing patents specifically utilizes a swivel mechanism for enabling a transverse beam to rotate to an out of square position during such cutting, which operation is to be avoided by cutting machine systems in accordance with the present invention.
Accordingly, there exists a clear need for shape cutting machines which perform in accordance with industrial standards and operate at higher cutting speeds but do not require extensive structural stiffening, sophisticated electronic control circuitry or precision rack and pinion drives.